How Can Multiple Units Cast Same Spell Simultaneously?

I have a group of units with a trigger ability that adds damage for a short period of time, but when I have them all selected and click the button, only one of them uses it. The base ability is Water Elemental. Is there any settings or something that I could do to change this behavior so that all units selected will use the ability?

You don't need JASS for this, check this thread in map development

Thanks, Lord Vexorian... very nice idea posted there.